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Energy Dissipation and Recovery in the Context of Silicon Production: Exergy Analysis and Thermoelectricity

Børset, Marit Takla
Doctoral thesis
Åpne
Fulltext not available (Låst)
Permanent lenke
http://hdl.handle.net/11250/2375036
Utgivelsesdato
2015
Metadata
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  • Institutt for kjemi [1064]
Sammendrag
The metallurgical industry is power intensive and produces not only metals but

also large amounts of excess heat. To increase the energy efficiency in this industry

is thus of major interest, from both an environmental and an economical point of

view.

Exergy analyses were carried out for two industrial silicon furnaces (A and B), in

operation at two different plants, and for a theoretical description of the silicon

furnace. Furnace A was evaluated for two choices of carbon raw material mixtures

and furnace B was assessed in the absence and presence of power production from

waste heat in the off-gas system. The theoretical process was used to estimate what

high exergy efficiency means in numbers and to explore the characteristics of the

exergy flow in such an ideal process.

The overall exergy efficiencies were 0.30-0.33 for the industrial silicon furnaces with

no power recovery. For furnace B, the overall exergy efficiency was estimated to

increase from 0.33 to 0.41 with power recovery from thermal energy in the offgas.

The silicon yield exergy indicator is a measure for the performance of furnace

operation that coincides with the specific power consumption. The theoretical

process provides the upper limit of 0.51 for this indicator while for the industrial

furnaces (no power recovery) values were 0.38-0.41. Additional exergy introduced

as volatiles and through the consumption of electrodes accounted for 8-11 % of

the total exergy destruction in the industrial silicon furnaces. Exergy destruction

due to combustion of by-product gases, exergy lost with the furnace off-gas and

ohmic losses in the power supply system were other major contributors to the

thermodynamic inefficiency of silicon furnaces.

Thermoelectric generators are scalable, simple and adaptable systems for power

generation from various heat sources. A 0.25 m2 thermoelectric generator (TEG),

based on bismuth-tellurium technology, was constructed and implemented at a

silicon plant to study power generation from heat available during casting of silicon.

The on-site measurements and a mathematical model were combined to explore the

potential further. The measured peak power was 160Wm−2 and the corresponding

maximum temperature difference across the modules was 100 K. For a twofold

increase of the heat transfer coefficient on the cold side, and by moving the generator

closer to the heat source, the power output was predicted to reach 900 W m−2.

By tailoring the design of the TEG to the conditions encountered in the industrial

facility, it is possible to generate more power with less thermoelectric material.

Guidelines were provided on how to design thermoelectric systems to maximize the

power generation from waste heat released from silicon during casting.

Electrochemical cells with molten carbonates and gas electrodes were studied theoretically

and experimentally for recovery of heat in the temperature range 400-

800 ◦C. The theory of non-equilibrium thermodynamics was used to describe the

energy conversion in the system. The measured Seebeck coefficients of these cells

were in the range 0.88 to 1.25 mV K−1. The larger value applies to a nearly equimolar

mixture of lithium and sodium carbonate in dispersion with magnesium oxide.

These cells can also exploit off-gases with concentration of carbon dioxide different

from air, by adding a concentration cell operating on off-gases and air. The series

construction has the potential to offer a power density at matched load conditions

in the order of 0.5 kW m−2.

The exergy analyses mapped the current status of the major exergy flows, exergy

destruction and exergy losses in the silicon furnaces. These analyses quantified

the potential for (thermodynamic) improvements, contributed to the fundamental

understanding of energy conversion in the silicon furnaces and gave estimates for

upper limits to exergy efficiency in this process. Thermoelectric generators are a

class of systems with potential for recovery of heat from metallurgical processes.

The cheap materials and the large Seebeck coefficient, in combination with a good

overall electric conductivity, make the molten carbonate cells suitable candidates

for electrochemical energy conversion in the future.
Utgiver
NTNU
Serie
Doctoral thesis at NTNU;2015:315

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